1. Field of the Invention
This invention pertains generally to an optical downconverter and more specifically to an invention for improving the image rejection capability of image rejection downconverters.
2. Description of the Related Art
There are two primary types of image rejection systems that provide significant frequency translation; those which use a digital phase modulator to produce a serrodyne phase modulated waveform and those which split a received signal into two parts and then recombine them in a way which eliminates the unwanted image and carrier frequencies.
Optical image rejection mixers that utilize the serrodyne method of frequency translation apply a phase modulated sawtooth waveform to the optical signal, thereby causing the frequencies to shift. The achievable image rejection is limited by the number of discrete bits that can be implemented by the digital phase modulator when approximating the sawtooth waveform. This limits the image rejection to approximately 25 dB.
In the case of microwave image rejection mixers, the achievable image rejection is limited by the need for near perfect amplitude and phase control. As shown in FIG. 1, a received radio frequency (RF) signal 12 is first divided into, two RF signals 16 and 18 in an in-phase power divider 14 mixed with an electromagnetic signal 22 from a local oscillator, (commonly referred to as LO IN) which is split into two signals 24 and 26 in a first 3 dB 90xc2x0 hybrid combiner 28. The mixers 32 and 34 mix the pair of signals from the local oscillator 26 and 28 with the pair of input RF signals 16 and 18 to produce a pair of respective RF signals 36 and 38 having a 90xc2x0 phase shift between the two component signals 36 and 38, which are recombined in a second 3 dB 90xc2x0 hybrid circuit 42 to produce an output intermediate frequency (IF) 44 for use by other electronic components (not shown). Exact amplitude and phase matching with broad band signals is nearly impossible with strictly microwave components since the frequency response of each of the components varies. A typical device, with a 3xc2x0 phase error and a 0.25 dB amplitude imbalance upon recombination, is limited to about 30 dB of image and carrier rejection. Further details on serrodyne frequency shifting is found in Johnson et al., SERRODYNE OPTICAL FREQUENCY TRANSLATION WITH HIGH SIDEBAND SUPPRESSION, J. of Lightwave Tech.; Vol. 6, pg. 109, 1988.
An optical image rejection downconverter with  greater than 60 dB of image rejection is a recently developed device that utilizes an electronic mixer to upconvert signals into the passband of a bandpass filter followed by optical downconversion of the filtered signals into the desired output band. This system developed by Ward et al. for which patent protection is being sought in the Navy Case No. 79,800, U.S. Patent Application Serial Number (not yet assigned) filed on Jul. 17, 2000, entitled IMAGE REJECTING MICROWAVE PHOTONIC DOWNCONVERTER is an improvement over the previous presented optical and electronic mixers. The system exhibits more than 60 dB of image rejection; however, it is limited by the production of unwanted harmonics by the upconverting mixer. The key to eliminating the spurious signals is to change which sideband is filtered. In Ward et al. the system is designed to operate with the local oscillator (LO1) at a frequency below the passband of the band pass filter (BPF), causing the negative (lower sideband) frequencies of the received RF signal to be shifted into the bandpass of the BPF. With this technique, unwanted frequencies are converted into the passband of the bandpass filter and as a result appear at the output of the system. These unwanted frequencies produce spurious signals in the system output which are only 25 dB below the desired signal power, severely limiting the sensitivity of the system.
A object of the invention is a device to increase the amount of image rejection while decreasing the power in harmonic spurious radiations to approximately 60 dB below the desired signal power in an image rejection device.
Another object of this invention is to provide a device for upconverting broadband electrical signals, filtering out image frequencies in the electrical domain and then downconverting the desired frequency using optical techniques.
Another object of this invention is to obtain a frequency shifted microwave signal utilizing electro-optic modulators and bandpass electronic filters.
Another object of this invention is to provide an analog device that does not rely upon phase or amplitude matching to achieve image rejection.
These and other objects are accomplished by a remotable, ultrawide band optical image rejection downconverter using sub-carrier modulation techniques without concern for image frequency interferences in the shifted signal, thereby allowing telecommunications systems to downconvert densely multiplexed communications channels into a low frequency band where conventional electronics can perform signal-processing functions. Further, this invention has the image rejection ( greater than 120 dB) to provide unambiguous signals for direction finding applications and exhibits an efficient image that permits multi-octave microwave frequency reception and compression. This invention is intrinsically remoteable, and due to the various optical and electrical components proves to be very useful and practical in numerous fiber optic and antenna systems.